This invention relates to the design of shearheads in the reinforcement of junctions between for example, vertical columns and horizontal floors, where at least the floor is formed in reinforced concrete. A shearhead is a separately definable structure embodied in the concrete at the junction which serves to spread the load of the floor on the respective column and thereby reduce the effect of the vertical forces; i.e., reduce the stress in the slab concrete by increasing the critical shear perimeter around the column.
It has long been recognised that supporting an horizontally extending body or slab on one or more vertical columns imposes bending and shear forces on the slab at the junctions with the columns which must be withstood by the structure of the slab or resisted by specific design features at the respective junction. Traditionally, the cross-section of the columns has been enlarged at least at the junction either wholly or locally at the junction by the introduction of flared column heads. Alternatively or additionally, the depth of the slab has been increased in the vicinity of the column to reduce the stresses arising particularly from the local shear forces. However, in modern building design it has become desirable to eliminate such obstructions or modifications to the slab, and provide clear substantially perpendicular junctions between each column and the lower uniform surface of the slab. It is also desirable to be able to use columns of smaller cross-section for a given slab loading or to increase the span of the slab for a given column size or cross-section. A number of shearhead designs have been proposed to these ends, and examples of such proposals are disclosed in European Patent Specification Nos. 0079413 and 0128994. This subject has also been considered in the following papers published by the American Concrete Institute:
1. Corley, W. G., and Hawkins, N. M., "Shearhead Reinforcement for Slabs," ACI Journal, Proceedings V. 65, No. 10, October 1968, pp. 811-824.
2. Hawkins, N. M., and Corley, W. G., "Moment Transfer to Columns in Slabs with Shearhead Reinforcement", Shear in Reinforced Concrete, Special Publication SP-42, American Concrete Institute, Detroit, 1974, pp. 847-879. Also, Research and Development Bulletin RD 037.01D, Portland Cement Association.